The intent of this research project is to extend our on-going analysis of host parasite factors in leishmaniasis by examining in further detail how Leishmania donovani achieves its state of intracellular parasitism. To accomplish this principal long-term objective, we will examine in both in vitro and in vivo models particularly key immunopathogenic aspects of the host cellular immune response to L. donovani infection and the mechanisms utilized by this pathogen to evade killing and parasitize the host target cell, the macrophage. The specific aims of this project are to (1) establish the kinetics of replication or destruction of L. donovani within human mononuclear phagocytes and mouse spleen macrophages, (2) utilize these peripheral and visceral phagocytes to determine the role of soluble T lymphocyte products in activating host macrophages to kill L. donovani and characterize the active components of these lymphokines, (3) identify the molecular basis of the oxygen-dependent and -independent antileishmanial mechanisms of activated human macrophages and determine methods to enhance the effects of these mechanisms, (4) ascertain if L. donovani contributes to its own intracellular survival by directly inhibiting macrophage antimicrobial mechanisms or modifying the phagolysosome's milieu, or indirectly by inducing parasitized macrophages or host lymphocytes to secrete inhibitory factors, (5) define the immunoregulatory mechanisms by which suppressor macrophages and T cells induced by L. donovani infection exert immunosuppressive effects and develop methods to reverse this activity, and (6) apply the findings derived from aims 1-5 by testing the therapeutic efficacy of selected pharmacologic and immunologic agents. These studies are designed to address key but as yet unresolved immunopathogenetic issues in leishmaniasis. We anticipate that this detailed analysis of the protozoan itself, the target host cell, their interaction, and methods to either enhance parasite susceptibility to killing or augment the effects of the cell-mediated immune response will provide the basis from which to rationally approach the pharmacologic and/or immunologic management of patients infected with these world-wide protozoan pathogens.